In light of the evermore stringent emissions regulations that are planned to take effect over the next few years, including California Low Emission Vehicle II (LEV II), Federal USA EPA Tier 2 and European Union EU-IV, pre-catalyst engine-out HC emissions, especially during cold start and warm-up, are attracting significant efforts in research and development. This is due in large part to the fact that as much as 80 percent of the total hydrocarbon emissions produced by a typical, modern light-duty vehicle during the Federal Test Procedure (FTP) can occur during the first 120 seconds of the test.
These high levels of emissions are largely attributable to cold engine and exhaust component temperatures. Specifically, cold engine components necessitate fuel-rich operation, in which the excess fuel is used to compensate for the portion of fuel that has attached to the walls of the intake system and combustion chamber and, thus, is not readily combusted. In addition, a cold three-way catalyst cannot reduce a significant amount of the unburned hydrocarbons that pass through the engine during cold-start. As a result, high concentrations of unburned hydrocarbons are emitted from the tailpipe. It is understood that the over-fueling associated with excessive hydrocarbon emissions during cold-start could be substantially eliminated through the use of vaporized rather than liquid fuels.
A variety of systems have been devised to supply fine liquid fuel droplets and air to internal combustion engines that work relatively well after engine warm-up. These systems either supply fuel directly into the combustion chamber (direct injection) or utilize a carburetor or port fuel injectors to supply the mixture through an intake manifold into a combustion chamber (indirect injection). In currently employed systems, the fuel-air mixture is produced by atomizing a liquid fuel and supplying it as fine droplets into an air stream.
In conventional spark-ignited engines employing port-fuel injection, the injected fuel is vaporized by directing the liquid fuel droplets at hot components in the intake port or manifold. Under normal operating conditions, the liquid fuel films on the surfaces of the hot components and is subsequently vaporized. The mixture of vaporized fuel and intake air is then drawn into the cylinder by the pressure differential created as the intake valve opens and the piston moves towards bottom dead center. To ensure a degree of control that is compatible with modern engines, this vaporizing technique is typically optimized to occur in less than one engine cycle.
Under most engine operating conditions, the temperature of the intake components is sufficient to rapidly vaporize the impinging liquid fuel droplets. However, as indicated, under conditions such as cold-start and warm-up, the fuel is not vaporized through impingement on the relatively cold engine components. Instead, engine operation under these conditions is ensured by supplying excess fuel so that a sufficient fraction evaporates through heat and mass transfer as it travels through the air prior to impinging on a cold intake component. Evaporation rate through this mechanism is a function of fuel properties, temperature, pressure, relative droplet and air velocities and droplet diameter. Of course, this approach breaks down in extreme ambient cold-starts, in which the fuel volatility is insufficient to produce vapor in ignitable concentrations with air.
The mandate to reduce air pollution has resulted in attempts to compensate for combustion inefficiencies with a multiplicity of fuel system and engine modifications. As evidenced by the prior art relating to fuel preparation and delivery systems, much effort has been directed to reducing liquid fuel droplet size, increasing system turbulence and providing sufficient heat to vaporize fuels to permit more complete combustion.
Given the relatively large proportion of unburned hydrocarbons emitted during startup, this aspect of light duty vehicle engine operation has been the focus of significant technology development efforts. Furthermore, as increasingly stringent emissions standards are enacted into legislation and consumers remain sensitive to pricing and performance, these development efforts will continue to be paramount. One general class of solutions employed to reduce engine startup emissions involves fuel vaporization. Key practical challenges to providing vaporized fuel include the fact that metering fuel vapor is problematic, and thus most approaches to reducing cold-start emissions focus on metering the fuel as a liquid and then vaporizing it. Heated fuel injector concepts with fuel heaters or vaporizers added on at the outlet of the injector generally suffer from poor atomization and fuel targeting once the heater is turned off. In addition, heated injector and heated impingement plates suffer from an intrinsic design challenge between minimizing the power required to the heating element and minimizing the vaporizer warm-up time. For practical purposes, the heating time associated with both heated injectors and heated impingement plates is too long unless excessive electrical power is supplied.
Also of interest to the future of the transportation sector is the supplementation or potential replacement of petroleum-based fuels. Alcohol fuels provide an attractive alternative to petroleum-based fuels for automotive applications since these fuels are renewable and derived from a number of sources, including those that are domestically available such as corn. Furthermore, alcohol is free of many of the technical barriers that have limited the market penetration of other alternative fuels for light-duty passenger vehicle applications.
Advantages associated with the use of alcohol fuels include the fact that they readily blend with petroleum-based fuel. As a result, alcohol fuels are compatible with the existing petroleum infrastructure, although some modifications are required. The ability to blend alcohol and petroleum-based fuels permit the gradual introduction of this alternative fuel and further allows for alcohol production to ramp up in accordance with demand rather than in anticipation of demand. Another advantage of alcohol fuels is that they are liquid at ambient conditions, eliminating the need for specialized storage and/or injection systems. Additionally, vehicular modifications required to accommodate alcohol fuels are relatively straightforward, with the cost being transparent to the consumer.
Despite these and other advantages associated with alcohol fuels, there are also distinct challenges associated with the use of neat alcohol fuels and alcohol/petroleum blends containing a high volumetric fraction of alcohol. One such challenge is cold starting an engine operating on a predominantly alcohol mixture. As is well known, alcohol fuels have a much lower volatility than gasoline and, as such, do not readily evaporate and subsequently ignite during cold-start and warm-up conditions.
Current approaches to addressing the challenge associated with alcohol-fueled engines often involve the use of an alcohol fuel sensor to provide feedback to the engine control unit (ECU). Within the ECU, the alcohol fuel sensor signal is used primarily for two purposes: 1) to determine whether or not a heat source should be used to vaporize the fuel upon cold-start and warm-up of the engine and 2) to adjust the fuel injection parameters for cold-start, warm-up and normal operation of the engine.
One particular solution to the aforementioned challenges associated with fuel vaporization in alcohol-fueled internal combustion engines involves the use of capillary passages to vaporize fuel. The use of capillary passages offers a number of distinct advantages over other techniques that are directed at supplying vaporized fuel to internal combustion engines. In particular, the high surface area to volume ratio of a capillary passage combined with the relatively low thermal mass associated with certain capillary structures result in fast warm up times (on the order of less than 0.5 seconds) and minimal power requirements (on the order of 150 watts per cylinder) necessary to achieve a desired heating profile. Yet another advantage of capillary passages used in fuel vaporization is that the capillary design may be integrated with the functionality of a conventional fuel injector so that a single injector can supply both liquid and vaporized fuel, depending upon the selected emission control strategy.
One form of a capillary passage-based fuel vaporizer is disclosed in U.S. application Ser. No. 10/284,180, filed on Oct. 31, 2002. In that application, a fuel system for use in an internal combustion engine is disclosed and a preferred form includes a plurality of fuel injectors, each injector including (i) at least one capillary flow passage, the at least one capillary flow passage having an inlet end and an outlet end, (ii) a heat source arranged along the at least one capillary flow passage, the heat source operable to heat a liquid fuel in the at least one capillary flow passage to a level sufficient to convert at least a portion thereof from the liquid state to a vapor state, and (iii) a valve for metering fuel to the internal combustion engine, the valve located proximate to the outlet end of the at least one capillary flow passage, a liquid fuel supply system in fluid communication with the plurality of fuel injectors, a controller to control the power supplied to the heat source of each of the plurality of fuel injectors to achieve a predetermined target temperature, the predetermined target temperature is operable to convert a portion of liquid fuel to the vapor state; means for determining engine air flow of the internal combustion engine, and a sensor for measuring a value indicative of degree of engine warm-up of the internal combustion engine, the sensor operatively connected to the controller; and wherein the portion of liquid fuel to be converted to the vapor state is controlled with reference to sensed internal combustion engine conditions to achieve minimal exhaust emissions.
The fuel system disclosed in application Ser. No. 10/284,180 is effective in reducing cold-start and warm-up emissions of an internal combustion engine. Efficient combustion is promoted by forming an aerosol of fine droplet size when the substantially vaporized fuel condenses in air. The vaporized fuel can be supplied to a combustion chamber of an internal combustion engine during cold-start and warm-up of the engine and reduced emissions can be achieved.
application Ser. No. 10/284,180 also discloses a method for controlling the fuel system and delivering fuel to an internal combustion engine for a fuel system including at least one fuel injector having at least one capillary flow passage, a heat source arranged along the at least one capillary flow passage, the heat source capable of heating a liquid fuel in the at least one capillary flow passage to a level sufficient to convert at least a portion thereof from the liquid state to a vapor state, and a valve for metering fuel to the internal combustion engine, the valve located proximate to an outlet end of the at least one capillary flow passage. The method includes the steps of determining engine air flow of the internal combustion engine, measuring a value indicative of degree of engine warm-up of the internal combustion engine, determining a portion of liquid fuel to be converted to the vapor state by the at least one capillary flow passage, the determining step employing the measured values, controlling power supplied to the heat source of the at least one fuel injector to achieve a predetermined target temperature, the predetermined target temperature is operable to convert the portion of liquid fuel to the vapor state so determined and delivering the fuel to a combustion chamber of the internal combustion engine and wherein the portion of liquid fuel to be converted to the vapor state is determined to achieve minimal exhaust emissions.
According to one preferred form described in application Ser. No. 10/284,180, the capillary flow passage can include a capillary tube and the heat source can include a resistance-heating element or a section of the tube heated by passing electrical current therethrough. The fuel supply can be arranged to deliver pressurized or non-pressurized liquid fuel to the flow passage. The apparatus can provide a stream of vaporized fuel that mixes with air and forms an aerosol that can be carried by an air stream, regardless of the flow path, into the combustion chamber.
As further described in application Ser. No. 10/284,180, the target temperature of the capillary is determined through the use of a control algorithm designed to achieve an appropriate target setpoint. The target setpoint is the ratio of the hot resistance of the capillary to the cold (unheated) resistance of the capillary (R/Ro). The ratio R/Ro, in turn, corresponds to a desired bulk capillary temperature. The duty cycle of the electronic fuel injector, as requested by an ECU, provides an indication of the target amount of fuel that should be supplied to the engine. The exhaust gas oxygen sensor provides an indication of the fuel that actually has been supplied by the injectors to the engine.
Despite the advances in the area of fuels systems for use in internal combustion engines, a need exists for a system capable of addressing the difficulties associated with fuel vaporization in engines operating on alcohol fuel or alcohol-gasoline fuel blends and methods for controlling such fuel systems.